MIT-developed ink can be printed into 3D structures that light up in the presence of predetermined stimuli.

The MIT team specialises in developing technologies based on responsive materials, gels and polymers, and has developed a variety of inks for 3D printing. They realised that genetically-engineered cells could form the responsive component of a hydrogel-based ink, but took a different route to realising this than previous researchers who have tried to use engineered mammalian cells.

“It turns out those cells were dying during the printing process, because mammalian cells are basically lipid bilayer balloons,” explained Hyunwoo Yuk, co-author of a paper on the research in Advanced Materials. “They are too weak, and they easily rupture.”

Different areas of the MIT team’s tree-like design light up if the skin has been exposed to the appropriate substance

Rather than mammalian cells, the team decided to use bacteria, which have thicker cell walls, capable of surviving being squeezed through a printer niozzle, and are more compatible with hydrogels.

Researchers from MIT’s mechanical engineering department, led by Prof Xuanhe Zhao, worked with bioengineers led by Timothy Lu, to determine the best hydrogel and nutrients to support bacteria engineered by Lu’s team. To light up in the presence of certain chemicals Zhao’s engineers customised a 3D printer to work with an ink based on a hydrogel containing pluronic acid.

“This hydrogel has ideal flow characteristics for printing through a nozzle,” Zhao said. “It’s like squeezing out toothpaste. You need [the ink] to flow out of a nozzle like toothpaste, and it can maintain its shape after it’s printed.”

The team used the ink to print a 3D layered structure onto a transparent elastomer patch. To illustrate the technique, they call this structure a “living tattoo” even though it is not printed into living skin and is therefore not in fact a tattoo.

“We found this new ink formula works very well and can print at a high resolution of about 30 micrometres per feature,” Zhao said. “That means each line we print contains only a few cells. We can also print relatively large-scale structures, measuring several centimetres.” The tattoo design was a tree whose branches were printed with cells sensitive to different substances. If the skin onto which the patch was stuck had been exposed to these substances, the corresponding branches lit up.

The team also engineered bacteria to communicate with each other; some were induced to light up only when they received a certain chemical signal from another cell. To test this, they printed one patch with filaments containing signal-producing bacteria, another with signal-receiving bacteria, and overlaid the latter on the former. The receiving bacteria only lit up when they overlapped and receive “input” from the signalling bacteria in the lower layer.

Near-term applications of this technology include flexible patches and stickers that will be able to detect environmental stimuli such as pollutants, changes in pH and temperature, all of which can be engineered into the bacteria. The ink could also be used in drug capsules and surgical devices, which would release therapeutic substances over time; this application could also use cells which have been engineered to produce the therapeutic substances, said fellow team member Xinyue Liu.

“We can use bacterial cells like workers in a 3D factory. They can be engineered to produce drugs within a 3D scaffold, and applications should not be confined to epidermal devices. As long as the fabrication method and approach are viable, applications such as implants and ingestibles should be possible.”

Future applications see the technique used to construct “living computers”, Yuk said. These could contain multiple cells communicating with each other and passing signals back and forth like transistors in a microchip. “This is very future work, but we expect to be able to print living computational platforms that could be wearable,” he said.